Write a readme file to accompany the scenario (containing a description, author information, and whatever else you think the player should know before he starts). Then use a program like
WinZip to zip all the required files together. Find some nice Civ2 websites, find out whether they want scenarios, and if so how you can submit them.
Submit the zip file, and hope they actually put it up for download.
But before you go about sending in your scenario, you'd better read those documents that have been recommended earlier in this thread.
The required files would be the your_scenario.scn, also the your_scenario.txt (if you wrote an intro text), the readme, and any of the text or image files you changed. Those files could be rules.txt, events.txt, city.txt, game.txt, labels.txt, units.gif, title.gif, terrain1.gif, terrain2.gif, cities.gif, icons.gif and people.gif (I think that's all of the relevant ones).
Important note: if you do start fiddling around with those text/image files, make sure you don't overwrite the originals. If you have one of the add-ons (that is, a Civ2 version higher than 2.62) you can put scenarios in a separate folder, so you won't mix up all the files.
You can check your Civ2 version number as follows:
- Start a new game or load a saved game (whatever, as long as you have a game running),
- Open the "Game Options" from the Game menu.
- Your Civ2 version is the title of this game options window.